1. Field of the Invention
This invention relates to an ultrasonic sensor unit, and in particular, an ultrasonic sensor unit used for adjustment of optical axes of vehicle headlamps.
2. Description of the Related Art
In recent years, for headlamps, high-intensity lamps have been coming into use from the standpoint of safety.
However, while the high-intensity lamp contributes much to safety, it has a drawback that the risk of dazzling oncoming vehicles is high. Hence, techniques of adjusting the optical axes of headlamps according to the inclination of a vehicle have been being developed.
For example, there has been developed an optical axis adjustment apparatus in which two ultrasonic sensors are arranged, for example at the front end of a frame of a truck, along the longitudinal direction of the vehicle. Their sending elements send out ultrasonic waves toward the road surface, and their receiving elements receive the ultrasonic waves reflected by the road surface, respectively. From the difference between the times of reception of these reflected waves, the inclination of the front part of the vehicle is detected, and the optical axes of the headlamps are adjusted according to the inclination detected. An optical axis adjustment apparatus of this type is disclosed in Japanese Unexamined Patent Publication No. 2003-118476, for example.
In the optical axis adjustment apparatus disclosed in this patent publication, the ultrasonic sensors are arranged in the positions near the road surface. Hence, there is a problem that when the vehicle runs, for example on a road with fallen snow, snow and water drops splashed by the vehicle in a low temperature adhere to the ultrasonic sensors as frozen snow, and cause deterioration in the performance of the ultrasonic sensors.
Generally, the ultrasonic sensors are contained in a case and the case is fitted to, for example the frame. In order to improve the performance of the ultrasonic sensors and expose the sending and receiving elements, horns are formed to protrude from the inner side of the bottom of the case, having a tapered opening having a diameter gradually increased toward the outside of the case. In this arrangement, the above problem is serious, since frozen snow easily becomes stuck in the horn.
In this arrangement, even if frozen snow does not become stuck in the horn, water drops may run down the wall of the horn and turn to ice near the boundary between the horn and the bottom (in other words, the open end of the horn). The frozen snow may adhere to this open end like icicles, interfere with the ultrasonic wave signals sent out and received by the ultrasonic sensors, and prevent the ultrasonic sensors from operating normally.
In order to solve this problem, it is conceivable to add a heating means to the case. However, how to arrange the heating means effectively is an issue.
This invention has been made to solve the problems discussed above. An object of the invention is to provide an ultrasonic sensor apparatus in which a heater is arranged effectively in a case containing ultrasonic sensors to melt frozen snow attached to the case assuredly to ensure the performance of the ultrasonic sensors.